Enable memory-mapped coverage instrumentation

Bug: http://b/194128476
Bug: http://b/210012154

- pass -runtime-counter-relocation flag, which is needed to enable
memory-mapped coverage on Linux/Android.

- Include '%c' specifier in -fprofile-instr-generate compiler flag to
default to memory-mapped coverage.

- Disable continuous coverage if instrumentation is on for bionic/libc
(http://b/210012154).

Test: Run few coverage tests on Forrest.
Change-Id: Ie3a912f66470fcd3ffc2ffd73371a4e1d2b15df3