[libc] Add malloc.h header defining mallopt (#110908)

This patch adds the malloc.h header, declaring Scudo's mallopt
entrypoint when built LLVM_LIBC_INCLUDE_SCUDO, as well as two
constants that ca

[libc] Add malloc.h header defining mallopt (#110908)

This patch adds the malloc.h header, declaring Scudo's mallopt
entrypoint when built LLVM_LIBC_INCLUDE_SCUDO, as well as two
constants that can be passed to it (M_PURGE and M_PURGE_ALL).

Due to limitations of the current build system, only the declaration
of mallopt is gated by LLVM_LIBC_INCLUDE_SCUDO, and the two new
constants are defined irrespectively of it. We may need to refine
this in the future.

Note that some allocators other than Scudo may offer a mallopt
implementation too (e.g. man 3 mallopt), albeit with different
supported input values. This patch only supports the specific case of
LLVM_LIBC_INCLUDE_SCUDO.

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