[libc] need a better way to handle `PREFER_GENERIC`

[SchrodingerZhu]

We use PREFER_GENERIC to flag some targets such that we can test objects compiled without even SSE2 support. However, due to the limitation of current cmake setup, this means that -D__LIBC_PREFER_GENERIC will be added to the default builds to those objects, which is quite unfortunate.

We need to have an approach similar to the following:

add_header_library(
  bitmask
  HDRS
    bitmask.h
  FLAGS
    PREFER_GENERIC=0
  DEPENDS
    libc.src.__support.common
    libc.src.__support.CPP.bit
    libc.src.__support.macros.properties.cpu_features
)

in which we can specify the default value of a flag.

[SchrodingerZhu]

cc @nickdesaulniers @lntue

[SchrodingerZhu]

Or we can do with current setup and change the flag to something like "MACHINE_DEPENDENT_OPT", "EXPLICIT_SIMD_OPT".