RISC-V: fix build issue with gcc 4.9.x

RISC-V: fix build issue with gcc 4.9.x

GCC should still build with GCC 4.8.3 or newer [1]
using C++03 by default. But a recent change in
RISC-V port introduced a C++11 feature "std::log2" [2].

Use log2 from the C header, without the namespace [3].

[1] https://gcc.gnu.org/install/prerequisites.html
[2] https://gcc.gnu.org/git/?p=gcc.git;a=patch;h=7caa1ae5e451e780fbc4746a54e3f19d4f4304dc
[3] https://stackoverflow.com/questions/26733413/error-log2-is-not-a-member-of-std

Fixes:
https://gitlab.com/buildroot.org/toolchains-builder/-/jobs/4202276589

gcc/ChangeLog:
	* config/riscv/genrvv-type-indexer.cc: Use log2 from the C header, without
	the namespace.

Signed-off-by: Romain Naour <romain.naour@gmail.com>