[ottf] Fix global pointer initialization in ottf_start.S
[ottf] Fix global pointer initialization in ottf_start.S

Linker relaxations must be disabled until the global pointer setup. This
commit fixes this issue and moves the initialization of the global
pointer to the start of `_ottf_start` for clarity.

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan