PAC FAQ fix: Read HCR_EL2 instead of clobbering with SCTLR_EL1 (#2259)

The sample code for setting up PAC is missing the read register call for
`HCR_EL2`.  This leads to clobbering HCR_EL2 with whatever the bits in
`SCTLR_EL1` were.

Author: droe

docs/FAQ.md

@@ -231,6 +231,9 @@ Taken from [#1789](https://github.com/unicorn-engine/unicorn/issues/1789).
     reg.crm = 0b0001;
     reg.op2 = 0b000;
 
+    err = uc_reg_read(uc, UC_ARM64_REG_CP_REG, &reg);
+    assert(err == UC_ERR_OK);
+
     // HCR.API
     reg.val |= (1ULL<<41);