qht: require a default comparison function

qht_lookup now uses the default cmp function. qht_lookup_custom is defined
to retain the old behaviour, that is a cmp function is explicitly provided.

qht_insert will gain use of the default cmp in the next patch.

Note that we move qht_lookup_custom's @func to be the last argument,
which makes the new qht_lookup as simple as possible.
Instead of this (i.e. keeping @func 2nd):
0000000000010750 <qht_lookup>:
   10750:       89 d1                   mov    %edx,%ecx
   10752:       48 89 f2                mov    %rsi,%rdx
   10755:       48 8b 77 08             mov    0x8(%rdi),%rsi
   10759:       e9 22 ff ff ff          jmpq   10680 <qht_lookup_custom>
   1075e:       66 90                   xchg   %ax,%ax

We get:
0000000000010740 <qht_lookup>:
   10740:       48 8b 4f 08             mov    0x8(%rdi),%rcx
   10744:       e9 37 ff ff ff          jmpq   10680 <qht_lookup_custom>
   10749:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 4ef95d8..6d6c51b 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -295,7 +295,7 @@
     uint32_t trace_vcpu_dstate;
 };
 
-static bool tb_cmp(const void *p, const void *d)
+static bool tb_lookup_cmp(const void *p, const void *d)
 {
     const TranslationBlock *tb = p;
     const struct tb_desc *desc = d;
@@ -340,7 +340,7 @@
     phys_pc = get_page_addr_code(desc.env, pc);
     desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
     h = tb_hash_func(phys_pc, pc, flags, cf_mask, *cpu->trace_dstate);
-    return qht_lookup(&tb_ctx.htable, tb_cmp, &desc, h);
+    return qht_lookup_custom(&tb_ctx.htable, &desc, h, tb_lookup_cmp);
 }
 
 void tb_set_jmp_target(TranslationBlock *tb, int n, uintptr_t addr)