[5.7] Revert 'Remove --gc-sections for all targets for now (#5708)'

[finagolfin]

Cherrypick of #5735

Explanation: lld 13 changed how --gc-sections works last year, which was then flipped on by default, and causes all Swift code on ELF platforms not to link with lld 13 and 14 anymore if stripping is enabled.

Scope: Only affects ELF platforms

SR Issue: #5698 and swiftlang/swift#60406

Risk: low, since it only affects ELF platforms with lld, with nobody else complaining since this was merged six months ago in #4135

Testing: I just built the last August 2 source snapshot of the 5.7 toolchain for Android AArch64, where lld 14 is the default linker, with this flag enabled back then and it worked well.

Reviewer: @keith

This depends on swiftlang/swift#60653 and swiftlang/swift-driver#1158 getting merged first.

[keith]

@swift-ci please smoke test

[finagolfin]

@tomerd, the relevant 5.7 driver pulls for lld linked above have not been merged yet, would you ping the relevant 5.7 manager to get them in?

[tomerd]

sry @buttaface, I will ask, should we revert?

[finagolfin]

Alright, they're in now, this is good.