[libc] remove MPFR and related tests in full build (#87693)

In full build mode, the fuzzing tests fail to build. This PR disabled MPFR related tests in full build
```
[2/4] Building CXX object projects/libc/fuzzing/stdio/CMakeFiles/libc.fuzzing.stdio.printf_float_conv_fuzz.dir/printf_float_conv_fuzz.cpp.o
FAILED: projects/libc/fuzzing/stdio/CMakeFiles/libc.fuzzing.stdio.printf_float_conv_fuzz.dir/printf_float_conv_fuzz.cpp.o 
/usr/bin/clang++ -DLIBC_NAMESPACE=__llvm_libc_19_0_0_git -I/home/schrodingerzy/Documents/llvm/llvm-project/build/projects/libc/fuzzing/stdio -I/home/schrodingerzy/Documents/llvm/llvm-project/libc/fuzzing/stdio -I/home/schrodingerzy/Documents/llvm/llvm-project/libc -isystem /home/schrodingerzy/Documents/llvm/llvm-project/build/projects/libc/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc...