let EXTRA_CXXFLAGS override -fsanitize=address in test/Makefile

[avionicharshit-byte]

the test suite cannot start on arm64 linux kernels built with a 39 bit virtual address space, raspberry pi os included. asan maps its allocator at a fixed 0x500000000000, the top user address there is 0x8000000000, so it aborts before the first test:

AddressSanitizer: CHECK failed: sanitizer_allocator_primary64.h:131 "((kSpaceBeg)) == ((address_range.Init(TotalSpaceSize, PrimaryAllocatorName, kSpaceBeg)))" (0x500000000000, 0xfffffffffffffff4)

there is no way to opt out today. EXTRA_CXXFLAGS sits before -fsanitize=address in CXXFLAGS, so -fno-sanitize=address loses. this moves $(EXTRA_CXXFLAGS) to the end. existing ci uses are unaffected, -m32 and -fno-exceptions are position independent and -std=c++17 already relies on last wins.

verified on a raspberry pi 4, aarch64, gcc 14.2.0. make CXX=g++ EXTRA_CXXFLAGS=-fno-sanitize=address test builds with zero warnings under the existing warning set, and the suite runs:

846 tests from 162 test suites ran
[  PASSED  ] 842 tests
[  SKIPPED ] 4 tests

the skips are environmental, 3 want CPPHTTPLIB_TEST_ISSUE_2431 and a sinkhole dns, 1 wants a comma decimal locale that is not installed.

this fixes nothing in httplib.h, arm64 and gcc both look clean.

[avionicharshit-byte]

the abi failure here isnt from this pr, its a one line reorder in test/Makefile with no library changes

abidiff builds against the last tag, which is v0.54.1, and 7d53a31 (#2575) added a Response param to
detail::encoding_type and Server::static_file_encoding after that tag was cut

master's own push run for that commit is red for the same two symbols
https://github.com/yhirose/cpp-httplib/actions/runs/33455962542

[yhirose]

@avionicharshit-byte thanks!