[PATCH 24/27] gdb.base/solib-weak.exp: Skip on all PE/COFF targets

The gdb.base/solib-weak.exp testcase sets a breakpoint on a function
that two shared libraries both provide, where one library defines it
as a weak symbol and the other as a strong symbol, and checks that GDB
picks the right copy.  This relies on a weak symbol in one shared
library being overridden by a strong symbol of the same name in
another, resolved at load time, which is an ELF dynamic-linking
behavior.

The test was already skipped on mingw, cygwin and pe, since that
override does not happen on Windows: PE/COFF resolves symbols at link
time, per module, so a weak symbol in one DLL is not overridden by a
strong one in another DLL.  GCC on mingw does accept
__attribute__((weak)) and binutils does emit PE weak externals, but
those are a link-time, single-module alias-with-fallback mechanism,
not the cross-DLL runtime override the test exercises, so the test
fails there.

The existing early return open-codes mingw/cygwin/pe and so did not
cover windows-msvc, whose triplet is
{x86_64,aarch64,i686}-pc-windows-msvc.  Use is_pecoff_target instead,
which covers windows-msvc too, and reword the comment to say why the
test does not apply rather than just that "weak symbols are not
meaningful".

Confirmed the test still fails on windows-msvc before this change.

Tested on:

x86_64-pc-linux-gnu
 x86_64-pc-cygwin
 x86_64-w64-mingw32
 x86_64-pc-windows-msvc

Change-Id: I5b12784d32ecd6b0595d721e28bd10a61a9f1901
---
 gdb/testsuite/gdb.base/solib-weak.exp | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/gdb/testsuite/gdb.base/solib-weak.exp b/gdb/testsuite/gdb.base/solib-weak.exp
index 319100e4a55..21449061e14 100644
--- a/gdb/testsuite/gdb.base/solib-weak.exp
+++ b/gdb/testsuite/gdb.base/solib-weak.exp
@@ -19,12 +19,13 @@
 
 require allow_shlib_tests
 
-# These targets have shared libraries, but weak symbols are not meaningful.
-if {([istarget *-*-mingw*]
-     || [istarget *-*-cygwin*]
-     || [istarget *-*-pe*])} {
-    return
-}
+# This test relies on a weak symbol in one shared library being
+# overridden by a strong symbol of the same name in another, resolved
+# at load time.  That is an ELF dynamic-linking behavior.  PE/COFF
+# resolves symbols at link time, per module, so a weak symbol in one
+# DLL is not overridden by a strong one in another DLL -- the scenario
+# this test exercises does not exist there.
+require !is_pecoff_target
 
 # This test uses GCC-specific syntax.
 if {![test_compiler_info "gcc-*"]} {
-- 
2.54.0