Fix illumos compile error in coreclr/runtime/amd64/AllocFast.S

[gwr]

Using gcc 13.3 configured as a cross-compiler:

$ /crossrootfs/x64/bin/x86_64-illumos-gcc --version
x86_64-illumos-gcc (GCC) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

And the GNU assembler that uses:

$ /crossrootfs/x64/bin/x86_64-illumos-as --version
GNU assembler (GNU Binutils) 2.42
Copyright (C) 2024 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `x86_64-sun-solaris2.11'.

There are compiler errors in src/coreclr/runtime/amd64/AllocFast.S

 coreclr/runtime/amd64/AllocFast.S: Assembler messages:
  coreclr/runtime/amd64/AllocFast.S:237: Error: missing ')'

[gwr]

Apparently some compilers/assemblers don't like a parenthesis expression there.
It might be safer to just use a simple constant and put the math in a comment above.