kbuild: split the build log of kallsyms

Currently, the build log shows KSYM + object name.

Precisely speaking, kallsyms generates a .S file and then the compiler
compiles it into a .o file. Split the build log into two.

[Before]

  GEN     modules.builtin
  LD      .tmp_vmlinux.kallsyms1
  KSYM    .tmp_vmlinux.kallsyms1.o
  LD      .tmp_vmlinux.kallsyms2
  KSYM    .tmp_vmlinux.kallsyms2.o
  LD      vmlinux

[After]

  GEN     modules.builtin
  LD      .tmp_vmlinux.kallsyms1
  KSYMS   .tmp_vmlinux.kallsyms1.S
  AS      .tmp_vmlinux.kallsyms1.o
  LD      .tmp_vmlinux.kallsyms2
  KSYMS   .tmp_vmlinux.kallsyms2.S
  AS      .tmp_vmlinux.kallsyms2.o
  LD      vmlinux

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>