[ARM] Incorrect relocation type for Thumb2 B<cond>.w

The Thumb2 conditional branch B<cond>.W has a different encoding (T3) 
to the unconditional branch B.W (T4) as it needs to record <cond>. 
As the encoding is different the B<cond>.W is given a different 
relocation type. 

ELF for the ARM Architecture 4.6.1.6 (Table-13) states that 
R_ARM_THM_JUMP19 should be used for B<cond>.W. At present the 
MC layer is using the R_ARM_THM_JUMP24 from B.W.

This change makes B<cond>.W use R_ARM_THM_JUMP19 and alters the 
existing test that checks for R_ARM_THM_JUMP24 to expect 
R_ARM_THM_JUMP19.

llvm-svn: 271997