Mangle symbols with a mangled name close to PDB limits with v0 instead of legacy mangling to avoid linker errors

[Fulgen301]

This is rust-lang/compiler-team#934

As PDB debuginfo has a 64KiB limit for symbol names, we use v0 mangling instead of legacy mangling for symbol names >= 65000 bytes if PDB is used. The cutoff number was chosen to leave some room for potential errors in the empirical measurement of the limit of 65521 bytes, as well as potential symbol prefixes and suffixes that are applied later, plus some generous extra space.

Tracking issue: #148429

[rustbot]

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(See the Target Tier Policy.)

[rustbot]

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[fee1-dead]

Might need some testing, but otherwise I don't have enough experience to review this.

@rustbot reroll