test: Run cc-msvc-builtin-1923 test with suitable target triple

The test added by commit 4ef8cb8 (test: Add case for
__builtin_assume_aligned declaration in MSVC header, 2020-04-03,
v0.3.3~2^2) uses the MSVC `__cdecl` calling convention specifier.
That is not supported by all target ABIs, so update the test to
run CastXML using the `x86_64-pc-windows-msvc` target triple.

Fixes: #165

test/CMakeLists.txt

@@ -400,7 +400,9 @@ castxml_test_cmd(cc-msvc-std-explicit --castxml-cc-msvc "(" $<TARGET_FILE:cc-msv
 castxml_test_cmd(cc-msvc-builtin-1800-E --castxml-cc-msvc "(" $<TARGET_FILE:cc-msvc> -msc=1800 ")" ${empty_cxx} -E -dM)
 castxml_test_cmd(cc-msvc-builtin-1900-E --castxml-cc-msvc "(" $<TARGET_FILE:cc-msvc> -msc=1900 ")" ${empty_cxx} -E -dM)
 castxml_test_cmd(cc-msvc-builtin-1900 --castxml-cc-msvc "(" $<TARGET_FILE:cc-msvc> -msc=1900 ")" ${input}/make_integer_seq.cxx)
-castxml_test_cmd(cc-msvc-builtin-1923 --castxml-cc-msvc "(" $<TARGET_FILE:cc-msvc> -msc=1923 ")" ${input}/assume_aligned.cxx)
+if(";${LLVM_TARGETS_TO_BUILD};" MATCHES ";X86;")
+  castxml_test_cmd(cc-msvc-builtin-1923 --castxml-cc-msvc "(" $<TARGET_FILE:cc-msvc> -msc=1923 --cc-define=_M_X64 ")" ${input}/assume_aligned.cxx)
+endif()
 castxml_test_cmd(cc-msvc-c-bad-cmd --castxml-cc-msvc-c "(" cc-msvc-c-bad-cmd ")" ${empty_c})
 castxml_test_cmd(cc-msvc-c-src-c-E --castxml-cc-msvc-c $<TARGET_FILE:cc-msvc> ${empty_c} -E -dM)
 castxml_test_cmd(cc-msvc-c-src-c-cmd --castxml-cc-msvc-c $<TARGET_FILE:cc-msvc> ${empty_c} "-###")