use jemalloc's default prefix

By default, jemalloc only uses a prefix on OS X, iOS and Windows where
attempting to replace the platform allocator is broken. The lack of a
prefix allows jemalloc to override the weak symbols from the C standard
library to replace the system allocator.

This results in a performance improvement for memory allocation in C
along with reduced fragmentation. For example, the time spent on LLVM
passes in the Rust compiler on Linux is cut by 10% and peak memory usage
is reduced by 15%. Even on platforms like FreeBSD where jemalloc is the
system allocator, this eliminates the inevitable external fragmentation
and performance hit caused by using two general purpose allocators.

Closes #18676