gcc: don't require .gcno for assembly compiled with coverage flags

[cstrahan]

Problem

Caching an assembly compile that carries the gcov/coverage flags aborts the build:

$ sccache cc --coverage -c foo.S -o foo.o
sccache: encountered fatal error
sccache: error: failed to zip up compiler outputs
sccache: caused by: failed to open file `"foo.gcno"`
sccache: caused by: failed to open file `foo.gcno`: No such file or directory (os error 2)

--coverage / -ftest-coverage set outputs_gcno = true, and in gcc::parse_arguments
the .gcno note file is then added to the expected outputs as required
(optional: false). For C/C++/ObjC the compiler does emit a .gcno, but an assembly
input (.s / .S) accepts the coverage flags and produces no .gcno. When sccache
packages the compiler outputs for caching it can't find the file and fails the whole
compile.

This only surfaces because assembly is cacheable: the "Assembly language support" change
(d9d2eb6) added Language::Assembler / Language::AssemblerToPreprocess, so assembly
compiles now go through output packaging. Before that they were CannotCache and ran the
compiler directly, so the missing .gcno never mattered. It's the same class of bug as
the missing-.gcno case in #2275.

Real-world hit: any project that builds a .S under -Db_coverage/--coverage
(e.g. breakpad's breakpad_getcontext.S) can no longer be built through sccache with a
coverage build.

Fix

Mark the .gcno output optional for assembly languages. C/C++/ObjC are unchanged — the
.gcno stays required there, so a genuinely-missing note file for those languages is still
an error.

let optional = matches!(
    language,
    Language::Assembler | Language::AssemblerToPreprocess
);
outputs.insert("gcno", ArtifactDescriptor { path: gcno, optional });

Test

Added test_parse_arguments_coverage_assembly_gcno_optional, asserting that
--coverage -c foo.S yields a gcno output with optional: true (the existing
test_parse_arguments_coverage_outputs_gcno continues to assert optional: false for
foo.cpp).

Verified end to end with a from-source build:

• sccache cc --coverage -c foo.S now succeeds and is cached;
• sccache cc --coverage -c foo.c still emits and caches foo.gcno (restored on a cache
  hit), i.e. no regression for C/C++ coverage.

Fixes #2275.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.46%. Comparing base (4ffa89b) to head (c2e8197).

Files with missing lines	Patch %	Lines
src/compiler/gcc.rs	96.42%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (4ffa89b) and HEAD (c2e8197). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (4ffa89b)	HEAD (c2e8197)
	14	5

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2845      +/-   ##
==========================================
- Coverage   76.14%   67.46%   -8.69%     
==========================================
  Files          72       72              
  Lines       39807    38625    -1182     
==========================================
- Hits        30313    26060    -4253     
- Misses       9494    12565    +3071     

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