Support for 32 bit linux builds and gcc 4.9

[eakoli]

Adds option to enable 32 bit linux builds when on a 64bit host.
Links librt explicit in test suites (needed by older gcc chains that don't automatically manage it.)

[Swatinem]

Nice!

We do test for 32-bit GCC7 already:

sentry-native/azure-pipelines.yml

Lines 21 to 23 in 2e73065

	CC: gcc-7 -m32
	CXX: g++-7 -m32
	TEST_X86: 1

sentry-native/tests/__init__.py

Lines 103 to 104 in 2e73065

	if sys.platform == "win32" and os.environ.get("TEST_X86"):
	configcmd.append("-AWin32")

Would be nice to adapt the code to also do the right thing in this case

[Swatinem]

what I meant concerning the CI setup: can you remove the explicit -m32 flag from the CI config, and rather update the test runner to pass down your newly introduced cmake flag when the TEST_X86 flag is set?