Reformat imath.c macro to remove -Wmisleading-indentation warnings.

Recent versions of gcc whine about the admittedly-completely-illegible
formatting of this macro.  We've not noticed for a few reasons:

* In v12 and up, the problem is gone thanks to 48e24ba6b.
(Back-patching that doesn't seem prudent, though, so this patch
just manually improves the macro's formatting.)

* Buildfarm animals that might have complained, such as caiman,
do not because they use --with-openssl and so don't build imath.c.

* In a manual run such as "make all check-world", you won't see the
warning because it gets buried in an install.log file.  You have to
do "make -C contrib all" or the like to see it.

I noticed this because in older branches, the last bit doesn't
happen so "check-world" actually does spew the warnings to stderr.
Maybe we should rethink how that works, because the newer behavior
is not an improvement IMO.

Back-patch down to 9.2, pursuant to newly-established project policy
about keeping out-of-support branches buildable.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com

contrib/pgcrypto/imath.c

@@ -129,8 +129,13 @@ do{T *u_=(A),*v_=u_+(N)-1;while(u_<v_){T xch=*u_;*u_++=*v_;*v_--=xch;}}while(0)
 #define CLAMP(Z) s_clamp(Z)
 #else
 #define CLAMP(Z) \
-do{mp_int z_=(Z);mp_size uz_=MP_USED(z_);mp_digit *dz_=MP_DIGITS(z_)+uz_-1;\
-while(uz_ > 1 && (*dz_-- == 0)) --uz_;MP_USED(z_)=uz_;}while(0)
+do{ \
+  mp_int z_=(Z); \
+  mp_size uz_=MP_USED(z_); \
+  mp_digit *dz_=MP_DIGITS(z_)+uz_-1; \
+  while(uz_ > 1 && (*dz_-- == 0)) --uz_; \
+  MP_USED(z_)=uz_; \
+}while(0)
 #endif
 
 #undef MIN