[Backtracing][Linux] Include declarations for gettid() and tgkill().

[al45tair]

Older glibc doesn't include these. Nor does Musl, it seems. The previous fix put the gettid() declaration too far down the file.

rdar://110417355

[al45tair]

Explanation: The previous fix was right but in the wrong place in the file so we still had the problem. Also, @MaxDesiatov pointed out a problem with Musl which will probably also affect older glibc, so fix that at the same time. Risk: Low. The code change only affects Linux. Original PR: #66486 Reviewed by: @MaxDesiatov Resolves: rdar://110417355 Tests: This fixes a Linux nightly build failure on platforms with older glibc.

[al45tair]

@swift-ci Please test

[MaxDesiatov]

@swift-ci test windows

[al45tair]

@swift-ci Please test Windows platform